When using idmap backend = idmap_ad, Winbind doesn't start. 2 log messages led me to the solution:
Using idmap backend 'idmap_ad'
Succesfully added idmap backend 'ad'
Could not load idmap backend 'idmap_ad'
the idmap backend uses it's value as filename when searching for the backend, however the backend registers as 'ad' instead of 'idmap_ad' So it won't load. SOlution is simple -> rename idmap_ad.so to ad.so and change the idmap backend to 'ad' ot the other way around (make idmap_ad register as idmap_ad). If you know where is the problem, that the workaround is easy, however it might be a good idea to correct it in the distribution.
Michael Trezzi

Jerry, I would like to commit that patch which finally stripts the idmap backend name. Those having used "idmap backend = idmap_rid" would have to change to "idmap backend = rid". idmap_ad, and the static plugins tdb and ldap are not affected. What do you think?

And to make an update as smooth as possible I'll take care to strip of idmap_ from any module we get in loadparm. I discussed this with Günther and he'll assign this bug to me as soon he submitted his changes.